FULL PAPER

Hexakis(alcohol)zinc Complexes

Claas Sudbrake,[a] Bodo Müller,[a] and Heinrich Vahrenkamp*[a]

Dedicated to Professor Lawrence F. Dahl on the occasion of his 70th birthday

Keywords: Zinc complexes / Alcohol ligands / Structure and bonding

Hexakis(methanol) and hexakis(ethanol) complexes of zinc distances correspond to those in [Zn(H2O)6]2+. Their watersensitivity can be explained by a gain in entropy and by awere prepared under strictly anhydrous conditions and

crystallized as perchlorate or hexafluorosilicate salts in space gain in polar interactions upon replacement of alcohol bywater ligands.groups of high symmetry. All their OH functions are engaged

in hydrogen-bonded cation–anion networks. Their Zn–O

Hexaaqua transition-metal cations are commonplace and Preparation and Identificationserve as reference points for the discussion of structures,spectra or ligand-field effects as well as being the natural The extreme sensitivity of the isolated complexes toward

water required them to be handled with more than the usualstarting materials in preparative coordination chemistry. Incontrast, their simplest organic analogues, the hexakis(alco- precautions. In contrast, at the beginning of the prep-

arations moisture posed no problems as the zinc reagentshol) complexes, are so uncommon that they are normallynot mentioned in textbooks or even in comprehensive were the hexahydrates of the zinc salts. Their dehydration

was performed with trimethyl or triethyl orthoformate inhandbooks. [1] The reason for this is their extremely labilenature which does not allow them to be exposed to even the corresponding alcohol, which produces the alcohol it-

self as the hydrolysis product. This procedure, which wastraces of the “better” ligand water.When it comes to synthetic usefulness this lability be- applied by van Leeuwen for compounds 1 and 2, [2] [3] was

found to be useful for all three types of hexakis(alcohol)zinccomes favourable. Accordingly, hexakis(methanol) or hexa-kis(ethanol) complexes have been proposed as starting ma- salts prepared here. Thus, from solutions of the hexahy-

drates of zinc perchlorate, tetrafluoroborate, and hexa-terials for other complexes with weakly coordinating li-gands, and for this purpose a number of them have been fluorosilicate in methanol or ethanol, after treatment with

a large excess of the corresponding trialkyl orthoformateprepared as salts of the divalent transition metals. [2] [3] Theywere fully characterized by analyses and spectra. From their (10220 fold), the known perchlorate 1b, the known tetra-

fluoroborates 2a, b, and the new hexafluorosilicates 3a, bUV/Vis spectra it could be concluded that the simple al-cohols have about the same position in the spectrochemical were obtained {[(ROH)6Zn](ClO4)2: 1a: R 5 Me; 1b: R 5

Et; [(ROH)6Zn](BF4)2: 2a: R 5 Me; 2b: R 5 Et;series as water. So far, only one hexakis(methanol)vanadium-(II) complex[4] and two hexakis(ethanol)cobalt(II) com- [(ROH)6Zn](SiF6): 3a: R 5 Me; 3b: R 5 Et}. They are

colourless crystalline solids. Their hygroscopicity does notplexes[5] have been subjected to crystal structure determi-nations. allow them to persist for more than a few seconds in the

normal atmosphere.Our interest in these complexes arose from our attemptsat modelling the zinc enzyme alcohol dehydrogenase. [6] Unambiguous characterizations of 1b, 3a and 3b were

achieved with the structure determinations described below.These studies required the investigation of zinc2alcohol,zinc2alkoxide, and zinc2aldehyde complexes. It turned out NMR spectra in deuterioacetone showed that the alcohol

ligands stay attached to the zinc ion in this solvent, seethat some of the latter are accessible starting from hexakis-(ethanol)zinc salts. [7] This fact, together with the limited Table 1. The observed chemical shifts referred to those of

the free alcohols in the same solvent are small, yet signifi-knowledge about hexakis(alcohol)zinc complexes, [2] [3]

prompted us to re-investigate their chemistry and to gather cant. Typical dependencies of the chemical shifts upon thenature of the counterions point to cation2anion interac-information on their structures.tions in solution. The wide spread of the OH NMR reson-ances indicates the presence of hydrogen bonding. The lat-[a] Institut für Anorganische und Analytische Chemie, Universität

Freiburg, ter can also be concluded from the IR absorptions in a KBrAlbertstraße 21, D-79104 Freiburg, Germany matrix (see Table 2), which for 1 and 2 roughly resembleFax: (internat.) 1 49-(0)761/203-6001E-mail: vahrenka@uni-freiburg.de those in nujol mulls. [2] [3] The ν(OH) bands around

Eur. J. Inorg. Chem. 1999, 200922012 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999 143421948/99/111122009 $ 17.501.50/0 2009

C. Sudbrake, B. Müller, H. VahrenkampFULL PAPER3500 cm21 are quite broad and of relatively low energy.Other noticeable features in the IR spectra include the OHbending vibrations around 1630 cm21 and the typical in-tense absorptions of the anions in the 100021100 cm21

range for ClO42 and BF4

2 and around 750 cm21 forSiF6

22.

Table 1. 1H-NMR data ([D6]acetone), δ [ppm]

CH3[a] CH2

[a] OH Figure 1. Cation of the methanol complex 3a; code: Zn and Hwhite, C light grey, O dark grey; bond lengths [A] and angles [°]:Zn2O 2.086(1), O2C 1.430(2); O2Zn2O 180.0, 87.26(3), and1b [2] 1.18 3.72 5.3992.74(3), Zn2O2C 125.5(1)2a 3.32 2.97

2b 0.97 3.48 4.723a 3.20 2.953b 0.98 3.43 3.66

[a] 3J values for the ethyl groups 6.927.0 Hz.

Table 2. IR data (KBr), [cm21]

ν (OH) δ (OH) ν (anion)

1b [2] 3460m 1620m 1088s2a 3530s 1629m 1054s2b 3510s 1636m 1031s3a 3520s 1635m 765s3b 3510s 1600m 738s

Figure 2. Cation of the ethanol complex 1b (and similarly 3b); code:Zn and H white, C light grey, O dark grey; bond lengths [A] andangles [°] for 1b: Zn2O 2.078(2), O2H 0.82(6), O2C 1.445(4),C2C 1.483(7); O2Zn2O 180.0, 86.9(1) and 93.1(1), Zn2O2CSolid-State Structures130.2(2), O2C2C 110.9(4)

Crystals of 1b, 3a and 3b were found suitable for an X-ray data collection at ca. 2100°C. All three compounds cause the O2H hydrogen atoms to occur in two groups of

three on opposite sides of the ellipsoid, cf. the top center ofcrystallize in rhombohedral or hexagonal space groups withthe zinc ions located on threefold axes in positions with Figure 1. Figure 2 shows that in both ethanol complexes

the C2C bonds of the ethyl groups are oriented similarlysixfold point symmetry. Thus, there is only one alcohol li-gand each in the asymmetric unit, and one Zn2O bond and roughly parallel to the threefold crystallographic axis,

arranging the ethanol ligands in two groups of three. Thelength and two O2Zn2O angles define the coordinationsphere. In the case of 3b a disorder problem of both the van der Waals contacts of the ethanol ligands are maxim-

ized in this arrangement, forming two hydrophobic islandscoordinated and a co-crystallized ethanol molecule affectsthe precision of the molecular details which therefore are separated from the polar center of the complex comprised

of the Zn2O and O2H bonds. The resulting and character-not discussed here. The general features of the complex cat-ion of 3b, however, are the same as those of 1b. istic barrel shape of the ethanol complex cations is also veri-

fied for the hexakis(ethanol)cobalt(II) complex. [5] It is remi-Figures 1 and 2 are combined views of the complex cat-ions, showing both the ball-and-stick and the space-filling niscent of the layered arrangement of the two cis-1,3,5-

cyclohexanetriol ligands in their ZnL2 complex. [9]displays. The symmetry of the complexes is close to ideallyoctahedral. The Zn2O bond lengths (average 2.08 A) are The separation of the hydrophilic and hydrophobic re-

gions in the complexes also has the consequence that thevirtually identical to those in the Zn(H2O)621 cation (aver-

age 2.09 A). [8] Hence, there is no lengthening due to steric alcoholic O2H bonds are exposed to the exterior of thecations. For instance in Figure 2 the white ball to the topcrowding of the six alcohol molecules, and the lability of

the zinc2alcohol complexes cannot be explained by a right of the zinc ion represents an OH hydrogen atom. Thisin turn means that these quite acidic hydrogen atoms areweakness of the Zn2O bonds.

While the ball-and-stick models in Figures 1 and 2 em- within contact distance of the anions in the lattice, facili-tating hydrogen bonding between cations and anions. Itphasize the symmetry of the complexes and their similarity

to the aqua complexes, their peculiarities can better be vis- turns out that this is the case in all three compounds. EachOH group is engaged in a reasonably strong (< 2.8 A) hy-ualized by space-filling models. Figure 1 shows that the

methanol complex has an ellipsoid-like shape. The separ- drogen bond. For 1b this means that three oxygen atoms ofeach perchlorate anion are hydrogen-bonded. For 3a andation of hydrophilic and hydrophobic parts therein is no-

ticeable from the orientations of the O2H bonds which 3b the number of fluorine atoms matches that of the OH

Eur. J. Inorg. Chem. 1999, 2009220122010

Hexakis(alcohol)zinc Complexes FULL PAPER

Figure 4. Hydrogen-bonded cation2anion contacts of one SiF6unit in 3a (O···F 5 2.66 A)

Figure 3. Hydrogen-bonded cation2anion contacts of one Zn-(EtOH)6 unit in 1b (O···O 5 2.79 A)

groups implying that there are also six hydrogen-bondinginteractions per SiF6

22 anion. The resulting networks aredifferent in all three cases, see Figures 325. Complex 3adisplays a one-dimensional array of hydrogen-bonded cat-ions and anions with triple junctions, complex 3b a single-layered two-dimensional array with double junctions, andcomplex 1b a double-layered two-dimensional array withsingle junctions.

DiscussionFigure 5. Hydrogen-bonded cation2anion contacts of one Zn-(EtOH)6 unit in 3b (O···F 5 2.75 A)In an alcoholic or water-free environment the hexakis(al-

cohol) complexes of zinc are stable as salts with noncoordi-nating anions. Even traces of water, however, lead to their acetone, as can be seen from the persistence of the com-

plexes in acetone solution. They compete with aromatic al-hydrolysis. The alcohols are weaker ligands for zinc than

Table 3. Crystallographic details

1b 3a 3b

Empirical formula C12H36Cl2O14Zn C6H24F6O6SiZn C12H36F6O6SiZn· C2H5OHMolecular mass 540.7 399.7 529.9Crystal size [mm] 0.2 3 0.2 3 0.1 1.2 3 1.0 3 0.9 0.6 3 0.4 3 0.4Space group P23 R23 P231cZ 1 3 2a [A] 8.720(3) 11.850(3) 8.650(1)b [A] 8.720(3) 11.850(3) 8.650(1)c [A] 9.359(1) 9.383(2) 17.450(3)V [A3] 616.3(2) 1141.1(5) 1130.7(3)dcalcd. [g cm23] 1.46 1.75 1.56µ(Mo-Kα) [mm21] 1.27 1.78 1.22hkl range h: 29 to 0 h: 210 to 15 h: 0 to 11

k: 29 to 0 k: 217 to 6 k: 29 to 0l: 212 to 12 l: 213 to 12 l: 223 to 23

Measured reflections 1185 1452 2059Independent reflections 995 878 915Observed reflections [I > 2σ(I)] 913 871 555Parameters 49 32 67Refined reflections 995 878 915R1 (obs. refl.) 0.059 0.022 0.093wR2 (all refl.) 0.163 0.061 0.286Residual electron density 11.0 10.3 10.9[e/A3] 21.5 20.6 21.4

Eur. J. Inorg. Chem. 1999, 200922012 2011

C. Sudbrake, B. Müller, H. VahrenkampFULL PAPERStructure Determinations: [11] The crystals were obtained directlydehydes, as evident from the synthesis of mixed alcohol/from the reaction solutions and used without drying in vacuo. Theyaldehyde complexes of zinc. [7] Thus, the alcohol complexeswere immersed in fluorinated polyether oil and immediately placedshould be valuable as starting materials for other complexesin the nitrogen stream of the diffractometer9s cooling system. Dif-of zinc with weakly coordinating ligands.fraction data were recorded at ca. 2100°C with the ω/2θ techniqueThe solid-state structures of the alcohol complexes revealon a Nonius CAD4 diffractometer fitted with a molybdenum tube

a contrast between the polar environment of the zinc ion as (Kα, λ 5 0.7107 A) and a graphite monochromator. Empirical ab-well as the O2H bonds and the hydrophobic nature of the sorption corrections based on ψ scans were applied. The structuresalkyl groups. The former results in hydrogen-bonded cat- were solved with direct methods and refined anisotropically withion2anion networks in which all OH functions are en- the SHELX program suite. [12] Hydrogen atoms were included with

fixed distances and isotropic temperature factors 1.5 times those ofgaged. The latter gives rise to a typical barrel shape of thetheir attached atoms. The OH hydrogen atom of 1b was locatedhexakis(ethanol)zinc cations.and refined freely. Parameters were refined against F2. The RThe Zn2O bonds in the complexes are as long as thevalues are defined as R1 5 Σ|Fo 2 Fc|/ΣFo and wR2 5Zn2O bonds in the Zn(H2O)6

21 ion which indicates that[Σ[w(Fo

2 2 Fc2)2]Σ[w(Fo

2)2]]1/2. Drawings were produced withthey are equally strong. Hence, the lability toward hydroly-SCHAKAL.[13] Table 3 lists the crystallographic data.sis cannot be explained by bond-strength arguments. How-

ever, the structures of the hexakis(ethanol) complexes offertwo arguments for the water sensitivity. One of them is a

Acknowledgmentsgain of entropy upon release of the alcohol ligands whichThis work was supported by the Deutsche Forschungsgemein-experience a high degree of order in the complexes. Theschaft.other is a gain of polar and hydrogen-bonded interactions

upon replacement of alcohol by water ligands, both be-tween cations and anions in the solid state and between [1] See Comprehensive Coordination Chemistry (Eds.: G. Wilkinson,

R. D. Gillard, J. A. McCleverty), Pergamon Press, Oxford,complex cations or the released alcohols and the solvent1988, vol. 2 (“Ligands”).molecules in aqueous solution [2] P. W. N. M van Leeuwen, Recl. Trav. Chim. Pays Bas 1967,86, 2472253.

[3] A. D. van Ingen Schenau, W. L. Groeneveld, J. Reedijk, Recl.Trav. Chim. Pays Bas 1972, 91, 88294, and references citedExperimental Section therein.

[4] F. A. Cotton, S. A. Duraj, L. E. Manzer, W. J. Roth, J. Am.General: General working and measuring procedures, see ref. [10]. Chem. Soc. 1985, 107, 385023855.The zinc salts were obtained commercially. The alcohols were ob- [5] I. Bkouche-Waksman, P. L9Haridon, Acta Crystallogr., Sect. Btained water-free and further dried with magnesium. [D6]Acetone 1977, 33, 11221; Bull. Soc. Chim. Fr. 1979, Partie I, 50253.

[6] B. Müller, A. Schneider, M. Tesmer, H. Vahrenkamp, Inorg.and petroleum ether (b.p. 50270°C) were distilled from and keptChem. 1999, 38, 190021907, and references cited therein.over 4-A molecular sieves. Nitrogen (99.999%) as inert gas was [7] B. Müller, H. Vahrenkamp, Eur. J. Inorg. Chem. 1999, 1172127.

dried with BTS catalyst. Reaction vessels, gas pipes etc. were heated [8] The Cambridge Crystallographic Data File contains 7 struc-in vacuum before use. 2 Complexes 1b, 2a, and 2b were prepared tures of compounds containing Zn(H2O)6

21, the InorganicCrystal Structure Base 15 such structures. The Zn2O(H2O) dis-according to the published procedures. [2] [3] The hygroscopicity oftances vary by about ±0.05 A around the average value ofthe complexes did not allow for C/H determinations. 2.09 A.

[9] J. Bebendorf, H. B. Bürgi, E. Gamp, M. A. Hitchman, A. Mur-Hexakis(methanol)zinc Hexafluorosilicate (3a): 6.00 g (19.02 mmol)phy, D. Reinen, M. J. Riley, H. Stratemeier, Inorg. Chem. 1996,of Zn(H2O)6(SiF6) in 25 mL of methanol was treated with 40 mL 35, 741927429.

(38.8 g, 366 mmol) of trimethyl orthoformate which caused heating [10] M. Förster, R. Burth, A. K. Powell, T. Eiche, H. Vahrenkamp,of the mixture and complete dissolution. After stirring for 24 h at Chem. Ber. 1993, 126, 264322648.

[11] The crystallographic data of the structures described in thisroom temp. and filtration, all volatiles were removed in vacuo. Thepaper were deposited with the Cambridge Crystallographicresidue was taken up in methanol and layered with petroleum ether Data Centre as supplementary publication no. CCDC-115889

to yield 5.80 g (76%) of 3a which was freed from solvent by a syr- (for 1b), -115887 (for 3a), and -115888 (for 3b). Copies of theseinge and dried in vacuo. 2 C6H24F6O6SiZn (399.7): calcd. Zn data are available free of charge from CCDC, 12 Union Road,

Cambridge, CB2 1EZ, U.K. [Fax: (internat.) 1 44-1223/336-16.36; found Zn 16.23.033; E-mail: deposit@ccdc.cam.ac.uk].

[12] G. M. Sheldrick, SHELX-86 and SHELXL-93, Programs forHexakis(ethanol)zinc Hexafluorosilicate (3b): Like 3a from 6.00 gCrystal Structure Determination, Universität Göttingen, 1986(19.02 mmol) of Zn(H2O)6(SiF6) and 40 mL (35.6 g, 240 mmol) ofand 1993.triethyl orthoformate in 25 mL of ethanol with 48 h of stirring. [13] E. Keller, SCHAKAL for Windows, Universität Freiburg, 1998.

Yield 4.20 g (46%) of 3b. 2 C12H36F6O6SiZn (483.9): calcd. Zn Received March 15, 1999[I99103]13.51; found Zn 13.75.

Eur. J. Inorg. Chem. 1999, 2009220122012